k = int(input())

a, b = 0, 0

for i in range(2):

   b += (k % 10) * (10**i)

   k //= 10

for i in range(2):

   a += (k % 10) * (10**i)

   k //= 10

if a == b:

   print(a)

else:

   print(max(a, b))